How does water move in the body?

• The cell membrane is semi-permeable

• Water can move freely

• Water is in equilibrium between cells and extracellular fluids (osmotic equilibrium)

• Ions and solutes are disequilibrium

• Osmosis water moves along its concentration gradient across a semi-permeable membrane

Distribution of solutes in the body fluid compartments

plasma

Intracellular fluid

Interstitial fluid

Distribution of solutes in the body fluid compartments

Ions and solutes are in disequilibrium

Ions and solutes are in disequilibrium

• Water can cross the cell membrane

Na+

K+

Na+

K+

proteins

Osmosis

• water moves along its concentration gradient across a semi-permeable membrane

• Water moves to dilute a solute

Osmosis

Osmotic pressure is pressure exerted to counter the movement of water to dilute something

Osmolarity

• Describes the number of particles in solution• Know this and the direction of water movement

can be predicted

• # of particles in 1 liter of solution

• Is expressed as osmoles/L, or OsM

• If very dilute: milliosmoles/L, or mOsM

• Human body, approx 300 mOsM

Osmolarity: number of particles in 1L

• 1 M glucose = 1 OsM glucose

• 1M NaCl = 2 OsM NaCl, because NaCl disassociates to 2 ions in solution.

Na+ Cl-

Compare the osmolarity of 2 solutions:

• Solution A

• 1 OsM glucose

• A is hyposmotic to B

• (A has fewer particles than B)

• Solution B

• 2 OsM glucose

• B is hyperosmotic to A

• (B has more particles than A)

• Solution C

• 1 OsM NaCl

• C is hypotonic to B

• (C has fewer particles/L than B)

Compare the osmolarity of 2 solutions:

• Solution B

• 2 OsM glucose

• B is hyperosmotic to C

• (B has more particles/L than A)

Compare the osmolarity of 2 solutions:

• Solution A

• 1 OsM glucose

• A is isosmotic to C

• Solution C

• 1 OsM NaCl

• C is isosmotic to A

Osmosis, the diffusion of water across the cell membrane, has consequences on cells

• After water leaves a cell, the volume changes (it can shrink)

Tonicity

• Describes how the cell volume will change in a solution

P is penetrating solute

N is nonpenetrating solute

Water moved into the cell to dilute the solutes.

• Cell gains volume in a hypotonic solution

• Cell looses volume in a hypertonic solution

• Cell keeps the same volume in an isotonic solution.

Tonicity indicates how the cell volume will change in a solution

• In a hypotonic solution, the cell has a higher concentration of a nonpenetrating solute than the solution, water moves in.

• In a hypertonic solution, the cell has a lower concentration of nonpenetrating solute than the solution, water leaves the cell

During intavenous injection:

• 0.9% (normal) saline isotonic

• D5--.9% saline (5% dextrose) isotonic

• D5W hypotonic

• 0.45% saline hypotonic

• Vs dehydration hypotonic

• Vs blood loss isotonic